ci(docker): fix repo mount path for pkgmgr as base layer of Infinito.Nexus

Standardize Docker/CI/test environments to mount pkgmgr at /opt/src/pkgmgr.
This makes the layering explicit: pkgmgr is the lower-level foundation used by
Infinito.Nexus.

Infra-only change (Docker, CI, shell scripts). No runtime or Nix semantics changed.

https://chatgpt.com/share/69427fe7-e288-800f-90a4-c1c3c11a8484

.github/workflows/test-virgin-root.yml

@@ -31,15 +31,15 @@ jobs:
           set -euo pipefail
 
           docker run --rm \
-            -v "$PWD":/src \
+            -v "$PWD":/opt/src/pkgmgr \
             -v pkgmgr_repos:/root/Repositories \
             -v pkgmgr_pip_cache:/root/.cache/pip \
-            -w /src \
+            -w /opt/src/pkgmgr \
             "pkgmgr-${{ matrix.distro }}-virgin" \
             bash -lc '
               set -euo pipefail
 
-              git config --global --add safe.directory /src
+              git config --global --add safe.directory /opt/src/pkgmgr
 
               make install
               make setup
@@ -50,5 +50,5 @@ jobs:
               pkgmgr version pkgmgr
 
               echo ">>> Running Nix-based: nix run .#pkgmgr -- version pkgmgr"
-              nix run /src#pkgmgr -- version pkgmgr
+              nix run /opt/src/pkgmgr#pkgmgr -- version pkgmgr
             '